Moles and Avogadro's Number
Although when the idea of the mole was developed we did not know how many particles were in a mole (just that it would be the same for any element or compound) we now have a number, which we call Avaogadro's number, after Amedeo Avogadro and out of respect for his hypothesis. So...
A mole is 6.022x1023 particles
To put that number in perspective, lets look at it several ways.
First, it's worth just writing out the number without scientific notation. Avogadro's number is 602,200,000,000,000,000,000,000. That's 602 sextillion, or 602 billion trillion.
Another way to think about the number is to imagine having that many of something. If you had a mole of softballs, you could build Earth. If you had a million plain M&M candies, it would cover the continental United States to a depth of more than 6 miles.
If you had a mole of pennies and distibuted them evenly, EVERYONE on Earth would be a millionaire.
Imagine that you were around 14 billion years ago (when the universe was formed--give or take billion) and that you started counting the atoms in a mole of carbon, one per second, every second of every day of every year since then. In those 14 billion years you would have counted less than 1/10th of 1% of the carbon atoms in that mole.
Most amazingly, if you had 6.022x1023 molecules of water, it would weigh 18 grams. Atoms and molecules are REALLY small.